The radar transmission in your ex's brain where the moment you are happy, and over them, a signal goes off in their brain to contact you and ruin everything.

Julie: today is the day my exes contact me.
Marisa: heh?
Julie: my ex matt called me last night
Julie: and my ex derek IMed me
Julie: neither i ever talk to
Marisa: of course
Marisa : they have happydar
Marisa: if ur not miserable over them
Marisa: they know it